This invention is directed to a new reaction type flux that was developed primarily to provide improved solderability of aluminum-zinc coated ferrous sheet and strip products. While such a flux has a broader range of application, i.e. soldering of aluminum, aluminum alloys, aluminum and aluminum alloy coated ferrous products, the further description will be directed towards its primary application.
For greater insight into the uniqueness of the aluminum-zinc coatings, particularly a coating consisting essentially of 25 to 70%, by weight, aluminum, balance zinc, reference may be made to U.S. Pat. Nos. 3,343,930 and 3,393,089, assigned to the assignee of the present invention. Very briefly, such coatings stand alone as coating protection for ferrous substrates as they combine the tenacity and corrosion resistance of an aluminum coating with the galvanic protection of a zinc coating. The development and emergence of this new aluminum-zinc coating required the development of new practices and materials to realize the full potential of such coatings for commercial products. The present invention is a part of the development program to achieve the potential inherent in these new aluminum-zinc coatings.
Certain restrictions or parameters for the flux were either apparent from the outset or were dictated by commercial considerations:
A. ELECTRIC IRON OR LOW TEMPERATURE SOLDERING DESIRED TO AVOID EXCESSIVE DISTORTION OF THIN SHEETS, AND MELTING OF ALUMINUM-ZINC COATING
B. GOOD CAPILLARY ACTION OR FLOW CHARACTERISTICS INTO JOINT
C. MINIMUM OF SURFACE PREPARATION PRIOR TO SOLDERING
D. ABILITY OF FLUX TO ATTACK, I.E. PENETRATE OR REMOVE OXIDE LAYER COMPOSED PRIMARILY OF ALUMINUM OXIDE
E. COMPATABILITY WITH SOLDER
F. SPEED OF SOLDERING
The investigation finally led to the flux as described in this specification and in the claims appended thereto.
During the early stages of the investigation, attempts were made to find a suitable organic flux. Organic fluxes are low temperature fluxes, and the least corrosive type of flux which means that post-cleaning is less of a problem as the flux residues are more easily removed. Further, for some applications, organic fluxes had been used to solder aluminum satisfactorily. Nevertheless, even with modified compositions the results were totally unsatisfactory. Soldering tests showed that wetting of the aluminum-zinc coating and capillary action of the solder into lap joints were nonexistent. The results of these trials indicated that a more corrosive flux was needed.
The failures here led to a consideration of the inorganic corrosive-type fluxes, composed of inorganic salts and acids. Various combinations of salts and acids were attempted but none of the formulations could penetrate or remove the oxide film on the aluminum-zinc coating.
Finally, reaction type fluxes were investigated. They are a special group of inorganic fluxes that were initially developed for aluminum. The primary difference between reaction inorganic fluxes and other inorganic fluxes is that the reaction fluxes consist of anhydrous salts and should not be dissolved in water as are most inorganic fluxes. The reaction does not occur in the presence of water. A commercial flux, typical of this group of fluxes is sold under the name "Alcoa 66A" and has a nominal composition comprising 9 parts by weight zinc chloride and 1 part by weight ammonium chloride.
Such a commercial flux was marginally successful in soldering an aluminum-zinc coated ferrous strip as such commercial flux broke up the oxide layer by reacting zinc chloride from the flux with the aluminum from the coating at a soldering temperature between 720.degree.-800.degree. F. As indicated previously, the results were marginal as the reaction temperature was in excess of 480.degree. F. and therefore too high to allow soldering to be performed with electric soldering irons. By being forced to use higher temperatures there is a high risk of melting the aluminum-zinc coating.
A microscopic examination of a 25% to 70% aluminum, balance zinc coating reveals the presence of a cored aluminum-rich dendritic phase in a zinc-rich interdendritic matrix structure. The solidus temperature of a typical coating containing 55 Al-Zn, as observed in a binary phase diagram of zinc and aluminum, is about 900.degree. F. However, the interdendritic region of the coating may be as low as the 720.degree. F. eutectic temperature. In the event melting of the zinc-rich matrix outside the joint occurs during the soldering the solder will be drawn out of the joint to the molten area causing poor bonding and impaired corrosion resistance. Thus, low reaction temperature fluxes are imperative for aluminum-zinc coated products.
Attempts were made to lower the reaction and soldering temperature of "Alcoa 66A" base composition by the addition of one or more common flux ingredients, such as aluminum chloride, lithium chloride, potassium chloride, sodium chloride, stannous chloride, and sodium fluoride. Many of the flux compositions were selected after examining available phase diagrams between various chlorides for compositions with low liquidus points. Despite such examination and subsequent testing, it became apparent that the reaction or soldering temperatures had not been lowered sufficiently to allow soldering with an electric iron.
The investigation continued with reaction-type fluxes but the further efforts were directed toward the use of stannous chloride as an alternative to zinc chloride. Ultimately a formulation of stannous chloride, ammonium chloride and sodium fluoride was selected, with the final work concentrated on the optimization of the three components. This final work revealed unexpected changes in performance with variations in the formulations. For example, in the Soldering Manual, American Welding Society, New York, N.Y. 1959, at page 34, there is taught a formulation comprising, by parts, 8.8 stannous chloride, 1 ammonium chloride, and 0.2 sodium fluoride. As will be demonstrated hereinafter, such formulation was found unacceptable for the desired purposes of this invention. It was discovered that by increasing the stannous chloride relative to the other ingredients, the soldering time was significantly reduced while the reaction temperature was lowered by over 35.degree. F.